Bluetooth: controller: conditionally reuse CC for ISR profiling

This commit adds the option of conditionally reuse an existing
CC register of the event timer for timer sampling used in ISR
profiling, in case SW tIFS switching is implemented based on the
event timer.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>
Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>